After a substantial price reduction and the introduction of 3G technology, Apple’s iPhone device has become best-selling handset at AT&T stores, eclipsing the BlackBerry Curve, according to new data released by Morgan Keegan analyst, Tavis McCourt.

The revamped iPhone 3G has “siphoned off sales from other smartphone vendors,” including Research In Motion, whose BlackBerry Curve device had been AT&T’s top seller for more than a year.

RIM has responded by curring the cost of the Curve smartphone from $149 to $99, in hopes of vaulting back to the #1 spot. This is a tough task, however, with Apple expected to sell 6.1 million iPhones this quarter, half of these in the U.S., according to McCourt.

After the iPhone and Curve, Samsung’s BlackJack 2, Palm’s Centro, and RIM’s BlackBerry Pearl are currently AT&T’s top sellers.